FBI releases video of Navy Yard gunman stalking prey

Chilling new surveillance footage released by the FBI shows the Washington Navy Yard shooter stalking co-workers inside the building's corridors, stealthily hunting down victims with a shotgun.

The FBI provided five different videos from cameras inside the facility that show Aaron Alexis, the 34-year-old gunman who was a former Navy reservist and IT contractor, enter the facility's parking garage and walk inside the hallways carrying the gun. He is dressed in black pants and a blue polo shirt, with an ID card around his neck.

Alexis, at times appears to avoid being seen by workers by darting behind corners in the hallways. He carries the shotgun low but raises it when he passes doors while he navigates the hallways.

In about one hour, Alexis shot and killed 12 people inside the building on Sept. 16 before he was killed in a shootout with police. He was armed with the shotgun and a Beretta handgun he is believed to have taken from one of his victims.

A senior Pentagon official told Fox News late Wednesday, "people in (the Defense Department) are furious that the FBI unnecessarily put out this video footage today. It only adds to the pain of the families and brought life back to a killer. It was gratuitous."

The FBI also released a timeline of events.

Alexis entered the facility's parking garage at 7:53 a.m. He made his way into Building #197 and went into the men's fourth-floor bathroom. At that time, he was carrying a backpack and clipboard, the FBI said.

He reemerged about five minutes later without the backpack or clipboard but he was carrying the shotgun. About a minute later, he shot his first victim in Building #197. The FBI said he made his way from the fourth floor to the third and appeared again on the first floor. He made his way back up to the third floor where he was killed by police, the FBI said.

Valerie Parlave, head of the FBI's field office in Washington, said Wednesday during a news conference that investigators are continuing to explore Alexis' background and motivations. She noted that Alexis had a well-documented history of mental health issues, many of which have been widely reported in the aftermath of the attack.

Parlave says Alexis held a delusional belief that he was being controlled by extremely low frequency waves, or ELF waves. Alexis had written "my ELF weapon" on a shotgun he used in the rampage.

He purchased the Remington 870 shotgun and ammunition at a gun shop two days before the shooting and appeared to be ready to die in during the attack, the FBI said. There is no indication that he was targeting specific workers, a statement from the FBI said.